Engine cold-start emissions released before an exhaust catalyst has been sufficiently warmed may lower vehicle exhaust quality. Accordingly, engine control systems may use various approaches to expedite attainment of an activation temperature (e.g., a light-off temperature) at the exhaust catalyst.
One example approach described in U.S. 2010/0005784 involves increasing exhaust gas temperatures. Therein, an exhaust backpressure valve (or exhaust throttle) is closed to increase an exhaust temperature and enable desulfination of an exhaust catalyst. However, the inventors herein have recognized potential issues with such an approach. Increasing exhaust back-pressure by closing the exhaust backpressure valve may increase the dilution of the cylinder charge with burned gases, resulting in unstable combustion under some conditions. Further, the diluted cylinder charge may limit the amount of spark retard that can be applied, thus limiting the temperature increase of the exhaust.
Another example approach is shown by Joergl et al in U.S. Pat. No. 7,617,678. Therein, the engine's air intake system includes a valve assembly having a valve and a housing with an inlet in fluid communication with an EGR cooler, an inlet in fluid communication with a charge air cooler, and an outlet in fluid communication with the engine. A position of the valve in the housing is adjusted with respect to the inlet and outlets so that an amount of exhaust gas recirculation can be adjusted during an engine cold-start, thereby elevating an exhaust temperature. At the same time, the need for a distinct EGR valve and an exhaust gas backpressure valve is reduced.
However, the inventors herein have realized that by independently controlling each of an exhaust gas backpressure valve (or throttle) and an EGR valve, synergistic benefits can be achieved that can expedite exhaust catalyst activation. Further, the synergistic benefits may outweigh the component reduction benefits. Further still, it may be more effective to transfer heat from exhaust before a pressure drop at exhaust valve opening (EVO) creates a corresponding temperature drop. In one example, this may be achieved by a method for expediting activation of an exhaust catalyst coupled to an engine comprising: during an engine cold-start, closing a post-catalyst exhaust throttle while diverting at least a portion of the throttled exhaust gas through an EGR cooler coupled upstream of the throttle. In this way, increased exhaust throttling and increased heat rejection from an EGR cooler may be synergized to rapidly activate an exhaust catalyst while also heating an engine.
As an example, during an engine cold start, while an engine temperature is below a threshold temperature, an exhaust throttle coupled downstream of an exhaust catalyst may be closed (or moved to a more closed position). By throttling the exhaust, heat transfer to the engine and exhaust catalyst can be improved. This can be attributed to two effects. First, the higher density of the (slower moving) exhaust gas due to the higher pressure improves heat transfer (per kilogram) of the exhaust flow. Further, the expansion to atmosphere post-catalyst drops the temperature below ambient temperature. In other words, a heat pump effect is achieved. This effect enables almost all the exhaust heat to be recovered without requiring the addition of a heat exchanger. By using a post-catalyst throttle, the time and temperature that a given mass of exhaust gas is in contact with engine parts is substantially increased, expediting catalyst activation. As such, the exhaust throttle may be intermittently opened in response to elevated exhaust backpressure to provide pressure relief. Further, an intake airflow may be controlled along with the exhaust throttle to limit engine output torque. An EGR valve positioned in an EGR passage coupled to the engine exhaust upstream of the exhaust backpressure valve (and downstream of the exhaust catalyst) may also be maintained closed during the cold-start. This enables the throttled exhaust to be directed through an EGR cooler of the EGR passage, thereby increasing an EGR outlet temperature. Since the heat exchanger of the EGR cooler is in communication with the engine coolant, by raising the EGR cooler outlet temperature, an amount heat rejected by the EGR cooler is increased, and an engine temperature can be rapidly raised. That is, the EGR cooler may be utilized to recover heat at high pressure. As such, this provides a more effective way of recovering latent heat from the water in the exhaust. In still further embodiments, intake and/or exhaust valve timings may be adjusted to reduce internal exhaust gas recirculation and increase combustion stability.
In this way, increased exhaust backpressure and increased heat rejection from an EGR cooler can be advantageously used to warm up an engine and an exhaust catalyst faster. The combination acts synergistically, enabling exhaust catalyst activation and engine warm-up to be expedited without compromising combustion stability, thus avoiding potential misfires. By rapidly heating the exhaust catalyst, cold-start exhaust emissions can be reduced. Additionally, by maintaining relatively low dilution even with the exhaust back-pressure valve closed, spark timing adjustments (e.g., spark retard) can be used to further increase the exhaust gas temperature.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.